The present invention relates to a semiconductor device having a MOS capacitor and to a method for manufacturing the same.
In general, high-frequency or high-speed semiconductor devices are manufactured in dimensions as small as fine processing techniques will permit. For this purpose, a self-alignment structure is adopted wherein an opening for diffusion also serves as a contact hole. A composite insulating film of an oxide film and a nitride film is also adopted. FIG. 1 is a cross-sectional view of a semiconductor device of this type. Such a semiconductor device is manufactured as outlined in the following steps.
In a semiconductor substrate 1 of one conductivity type are formed a first impurity diffusion region 2 of any conductivity type and a second impurity diffusion region 3 of the opposite conductivity type to that of substrate 1. An oxide film 4 and a silicon nitride film 5 are formed on the surface of the substrate 1 in the order named, and the composite film formed thereby is patterned. A thin oxide film 4a which is exposed and which is to constitute part of the capacitor is covered by a resist film. Predetermined contact holes are formed. A polycrystalline silicon layer 6 containing a diffusion source is deposited in one of the holes which are connected to the second impurity diffusion region 3. A third impurity diffusion region 7, for example, an emitter region, is formed in the second impurity diffusion region 3 by diffusion of impurity from the polycrystalline silicon layer 6. Thereafter, at least the thin oxide film 4a is covered with a resist film again. A contact hole 8 for a substrate-side electrode 10 of the capacitor as well as a contact hole 9 for an electrode 12 to be connected to the second impurity diffusion region 3 are formed. Finally, the metal electrodes 10, 11, 12 and 13 are formed.
With a conventional semiconductor device of such a structure and a method for manufacturing the same, if there is any defect such as a pin hole in the thin oxide film 4a which determines the capacitance of the MOS capacitor, a capacitor electrode 11 may extend into the first impurity diffusion region 2 (capacitor region) through the oxide film 4a resulting in a short-circuit or dielectric breakdown. The thinner the oxide film 4a is for the purpose of increasing the capacitor capacitance, the more notable this problem becomes. In addition to this, with conventional methods for manufacturing a semiconductor device, it is necessary to cover the oxide film 4a with a resist film in order to form contact holes. This results in even more frequent formation of pin holes in the thin oxide film 4a.
A method for forming an insulating film on a thin oxide film constituting a capacitor is known as a method for manufacturing a semiconductor device having a MOS capacitor without requiring a step for forming the resist film as described above. Thus as shown in FIG. 2, the silicon nitride film 5 including a silicon nitride film 5a remaining on the thin oxide film 4a is used as a resist film in order to form the contact hole 8 for the substrate-side electrode of the capacitor and to form the contact hole 9 for connection to the second impurity diffusion region 3. This method is the same as the method described with reference to FIG. 1, except that the silicon nitride film is used as a resist pattern.
However, with a semiconductor device manufactured in this manner, the insulating film of the MOS capacitor becomes thicker than the thickness of the oxide film 4a by a thickness corresponding to that of the silicon nitride film 5a. The capacitance C of the MOS capacitor is given by: EQU C=.epsilon..times.A/l
where .epsilon. is the dielectric constant, A is the area of the insulating film, and l is the thickness of the insulating film. From this relation, it is apparent that the capacitance of the capacitor is reduced by a value corresponding to the thickness of the silicon nitride film 5a which accounts for the increase in the thickness l of the insulating film. Therefore, the capacitance per unit area of a MOS capacitor having a silicon oxide film and a silicon nitride film both having a thickness of 1,000 .ANG. is 2/3 that of a MOS capacitor having a similar silicon oxide film of 1,000 .ANG. thickness as an insulating film.